The present invention relates to a fiber grating in which a diffraction grating (grating) exhibiting a periodical index difference is written in a core of an optical fiber, and a method and an apparatus for fabricating the same.
A conventionally known fiber grating of this type includes a grating written in a core of an optical fiber by a two-beam interference method or a phase mask method (for example, as disclosed in Japanese Laid-Open Patent Publication No. 6-235808, Japanese Laid-Open Patent Publication No. 7-140311 and Japanese Patent No. 2521708). In such a fiber grating, a fused quartz (core) doped with germanium (Ge) is irradiated with a coherent UV laser beam so as to generate (write) a Bragg grating by causing photo-induced refractive index change in an irradiated portion.
The optical fiber used as the target for writing is generally obtained by coating a core and a cladding with a coat layer of a UV curable resin or the like that is cured by absorbing UV. The coat layer formed on a write target portion is generally removed in writing the grating through the UV irradiation in employing either of the two-beam interference method and the phase mask method, and the portion from which the coat layer is removed is coated again after completing writing the grating.
When the coat layer is removed, however, the outer face of a non-coated fiber (the outer face of the cladding) is exposed to air, and there is a fear of degradation of the non-coated fiber proceeded through the exposure to air during the writing work, which can degrade the transmitting characteristic. In addition, the coat layer of the write target portion is removed not by mechanical means but by a chemical treatment for dissolving it with, for example, a chemical, so as not to damage the non-coated fiber. Therefore, the removal of the coat layer is troublesome, which is a factor in degrading efficiency in mass processing for writing the grating.
On the other hand, in order to effectively write the grating by UV irradiation through the coat layer without removing the coat layer, sensitivity of the core of the optical fiber, that is, the target for writing, to the photo-induced refractive index change (photosensitivity) may be possibly increased. As means for increasing the photo sensitivity, namely, means for causing comparatively large photo-induced refractive index change, it has been proposed that the core corresponding to the target for writing is doped with Ge in a higher concentration (a concentration for attaining a relative refractive index difference between the core and the cladding of, for example, 1.5 through 2.0%) than a general concentration (a concentration for attaining the relative refractive index difference of, for example, 0.9%), or that the core is loaded with hydrogen at a high pressure after doping it with Ge in a general concentration (as described in the transactions of the Institute of Electronics, Information and Communication Engineers, Vol. J79-C-1, No. 11, p. 415, November 1996).
When a fiber grating is fabricated by using a core doped with Ge in a higher concentration, however, the following problem occurs in connecting this fiber grating between general optical fibers so as to be used as a filter or the like: Since the core of the optical fiber connected (fused) to the fiber grating has the general specifications doped with Ge in the general concentration, the cores cannot be matched and a difference in the concentration of the doped Ge increases connection loss. On the other hand, when a core loaded with hydrogen at a high pressure is to be used for fabricating a fiber grating, the loaded hydrogen is diffused with time, and the core returns to the original state prior to the hydrogen loading in a comparatively short period of time (for example, several days). Therefore, the time period for forming the grating by the UV irradiation is limited to a rather short period, and additionally, it is difficult to control the wavelength in the UV irradiation because it is necessary to write the grating in consideration of the diffusion of hydrogen.
Furthermore, when a fiber grating where a grating has been written is expanded or shrunken due to an influence of temperature change or an externally applied tensile force, the reflective wavelength of the grating is shifted. Therefore, a fiber grating is required to have a mechanical strength characteristic and a stable temperature characteristic that it cannot be expanded/shrunken by temperature change.
The present invention was devised in consideration of these circumstances, and an object is providing a fiber grating in which a grating can be easily written without causing degradation of the transmitting characteristic. In addition, another object is providing a highly reliable fiber grating by stabilizing not only the transmitting characteristic but also the temperature characteristic.
Another object of the invention is providing a fiber grating and a method of fabricating the same in which the transmitting characteristic and the mechanical strength characteristic can be consistent with each other without spoiling improvement of productivity.
The fiber grating of this invention includes a core where a grating is written, a cladding for covering the core, and a UV transmitting resin layer for coating the outer face of the cladding, and the grating is written in the core by irradiating the core through the resin layer with UV of a specific wavelength band.
Since the coat layer covering the core and the cladding is made from a UV transmitting resin in this fiber grating, even when UV is irradiated through the coat layer, the UV transmits the coat layer to effectively irradiate the core, so that the grating can be written in the core. Accordingly, the grating can be written without removing the coat layer. Therefore, it is possible to avoid degradation of the transmitting characteristic accompanied by the removal of the coat layer as well as to easily fabricate the fiber grating with a process for removing the coat layer omitted.
The coat layer of the UV transmitting resin has a characteristic of transmitting at least UV of a specific wavelength band used for writing the grating. In this case, a preferred coat layer of a UV transmitting resin is concretely specified as follows: The preferred coat layer transmits UV of the specific wavelength band used for writing the grating, for example, a wavelength band of 250 nm through 270 nm. The UV transmitting resin may have a specific wavelength band (transmitting band) within a range between 250 nm and 350 nm. As a light source of the UV having such a wavelength band, one having high spatial coherency, such as solid laser, is preferably used.
The coat layer has a characteristic of curing by absorbing UV in a shorter wavelength band or a longer wavelength band than the specific wavelength band used for writing the grating.
In using the coat layer of this invention, the grating can be written by UV irradiation through the coat layer because it transmits UV in the specific wavelength band, and at the same time, the coat layer absorbs UV and cures in the formation thereof so as to work as a protecting coat of the optical fiber.
The core is preferably co-doped with at least Sn addition to Ge in an amount equivalent to that in the core of an optical fiber to be connected. The amount of Ge to be doped is preferably that for attaining a relative refractive index difference between the cladding and the core of approximately 0.9%, while the concentration of Sn is preferably 10000 through 15000 ppm.
The co-doped Sn (tin) stationary increases the photo-induced refractive index change of the core as compared with a core doped with Ge alone in a general concentration. As a result, the reflectance attained by irradiating with the UV can be increased as compared with that of the core doped with Ge alone in the general concentration. Specifically, a specific wavelength (Bragg wavelength) xcexB reflected by a grating is represented by the following formula (1), and the reflectance RB for reflecting light of the Bragg wavelength is represented by the following formula (2):
xcexB=2xc2x7nxc2x7Pxe2x80x83xe2x80x83(1)
n: Effective refractive index
P: Grating pitch
RB=tanh2 (xcfx80xc2x7Lxc2x7xcex94nxc2x7xcex7/xcexB)xe2x80x83xe2x80x83(2)
L: Grating length
xcex94n: Refractive index modulation
xcex7: Propagation optical energy included in a core region
In the case where the core co-doped with Sn is irradiated with UV, the refractive index modulation xcex94n is larger than in the cored doped with Ge alone in the general concentration, resulting in increasing the reflectance RB. Accordingly, by forming the coat layer from the UV transmitting resin, not only the grating can be effectively written without removing the coat layer but also the co-doped Sn can increase the reflectance of the written grating.
In addition, since the concentration of the doped Ge is equivalent to that in the core of an optic al fiber to be connected, connection loss can be prevented from increasing even when the fiber grating is connected to the optical fiber with the general specifications. Moreover, since the reflectance can be stationarily increased by co-doping Sn without conducting the high pressure hydrogen loading, the restriction in the time period for forming a grating as in the case where the high pressure hydrogen loading is conducted can be avoided.
The mechanism of the photo-induced refractive index change through UV irradiation has not been made clear yet although various ideas including the following have been proposed: An idea based on the Kramers-Kronig mechanism that the refractive index change is caused due to the bond between a Ge atom and SiO2 (quartz glass) changed through the UV irradiation; an idea based on a compression model that a glass bond cut through the UV irradiation causes collapse of the glass structure so as to increase the density, resulting in increasing the refractive index; and an idea based on a dipole model. Also, the mechanism of increase of the photo-induced refractive index change by co-doping Sn has not been made clear. The present inventors have, however, variously examined and tested materials to be doped in a core, premising that the high pressure hydrogen loading, which requires troubles and rather long time (for example, of two weeks) and is restricted in the time period for forming a grating, is not used and that the amount of Ge to be doped is set equivalently to that of the general specifications for avoiding the connection loss, resulting in finding that the reflectance can be increased by co-doping Sn as described above without causing the conventional problems.
Furthermore, the coat layer of the UV transmitting resin is preferably covered with a secondary coat layer after writing the grating. The secondary coat layer is formed from a material having a negative coefficient of linear expansion so as to cancel the expansion, caused by temperature change, of a glass part of the optical fiber having a positive coefficient of linear expansion. The secondary coat layer having a negative coefficient of linear expansion is formed from, for example, a liquid crystal polymer (LCP). In using the LCP, even when the core and the cladding show an expanding tendency due to temperature change, for example, temperature increase, the secondary coat layer shows a shrinking tendency, thereby suppressing canceling the expanding tendency of the core and the cladding. As a result, the fiber grating is never expanded/shrunken by the temperature change but remains in the state prior to the temperature change, namely, the stability against the temperature change is improved. Thus, the shift of the reflective wavelength derived from the expansion/shrinkage can be prevented and suppressed, so as to definitely keep the constant reflecting function. In this case, between the core and the cladding showing the expanding tendency and the secondary coat layer showing the shrinking tendency, the primary coat layer of the UV transmitting resin adhered onto the both interfaces works as a buffer layer for canceling the expansion/shrinkage. Also, by forming the secondary coat layer not only on the portion for writing the grating but also over the entire length of the optical fiber constituting the fiber grating, the stability against the temperature change can be improved as well as the mechanical strength characteristic against externally applied tension can be improved.
In the method of fabricating a fiber grating of this invention, a coat layer is formed in a comparatively large specific thickness for attaining the mechanical strength characteristic, and thereafter, UV is irradiated through the thick coat layer. Even in this case, when the irradiation energy is set within a specific range, it is possible to obtain a fiber grating having a transmitting characteristic with high reflectance without degrading the strength of the coat layer.
Specifically, after forming the coat layer of a UV transmitting resin in a large thickness so as to attain a mechanical strength characteristic equivalent to that of a coated fiber to be connected to the fiber grating, the UV irradiation is conducted at an irradiation energy density of at least approximately 1.5 through 4.0 kJ/cm2. In this case, even when the coat layer has a large thickness for attaining the mechanical strength characteristic equivalent to that of the coated fiber to be connected, a grating having a good transmitting characteristic with high reflectance can be written in the core, that is, the target for writing, by conducting the UV irradiation at the aforementioned irradiation energy density, and degradation of the strength of the glass part and the coat layer due to the UV irradiation can be minimized so as to keep the aforementioned mechanical strength characteristic. In addition, since the thick coat layer is formed before writing the grating, the fiber grating can be mass produced without spoiling improvement of productivity. Fiber gratings are classified into a short-period fiber grating for attaining refractive index modulation of a 0.5 xcexcm period for realizing back coupling to a core and a long-period fiber grating for attaining refractive index modulation of a 100 through 500 xcexcm period for realizing front coupling to a cladding, to both types of which the invention is applicable.
From the viewpoint of productivity, the coat layer is most preferably formed by single coating immediately after drawing a non-coated fiber. The xe2x80x9cthicknessxe2x80x9d of the coat layer may be determined so as to attain, after writing the grating, substantially the same mechanical strength characteristic as that of a general communication coated fiber to be connected to the fiber grating. Specifically, the thickness is at least approximately 30 xcexcm. In a non-coated fiber of 125 xcexcm, the coat layer is preferably formed in a thickness of 30 through 50 xcexcm, for example, of 37.5 xcexcm. According to this invention, since the coat layer is not removed in writing the grating, the strength of the optical fiber is minimally reduced, and hence, sufficient mechanical strength can be attained even when the coat layer has a thickness of 50 xcexcm or less.
Also, the xe2x80x9cirradiation energy densityxe2x80x9d of the UV is preferably set within the range between 1.5 and 4.0 kJ/cm2 in order to suppress increase of the Young""s modulus of the coat layer within a predetermined range. The UV irradiation may be conducted by any of various methods, for example, by repeating irradiation at a predetermined pulse frequency with a short pulse width and with a constant energy of each pulse; by continuous irradiation; and by intermittent irradiation with large energy at intervals.
The UV irradiation at such an irradiation energy density may be conducted with the UV converged by a cylindrical lens, and in this case, the entire coat layer is placed in a position within a beam pattern of the UV converged toward a focal point.
When UV is converged by the cylindrical lens, the irradiation density of the UV is highest at the focal point. However, when the UV irradiation is conducted through the coat layer of the coated fiber as the target for writing, the coat layer can be locally damaged, for example, burnt or changed in color (yellowed) by placing the coated fiber on the focal point. When the entire coat layer is placed within the beam pattern as described above, the coated fiber can be uniformly irradiated at the predetermined irradiation density without damaging the coat layer. In addition, from the viewpoint of shortening time for the irradiation process, the coated fiber as the target for writing is most preferably placed not only within the beam pattern but also in a position where the outer face of the coat layer of the coated fiber is internally in contact with the outer edge of the beam pattern. Specifically, the position for attaining the internal contact corresponds to a position closet to the focal point and a position for the entire coat layer placing within the beam pattern, and hence, the UV can be irradiated at the highest irradiation density in a range where the damage of the coat layer is avoided.
Moreover, when the core is loaded with hydrogen before the UV irradiation, the photo-induced refractive index change caused by the UV irradiation can be increased, resulting in forming a grating with further higher reflectance. Furthermore, when the core as the target for writing is co-doped with at least Sn in addition to Ge, the photo-induced refractive index change caused by the UV irradiation can be increased even if the concentration of the doped Ge is equivalent to that in a coated fiber to be connected, resulting in forming a grating with further higher reflectance.
In conducting the UV irradiation through the coat layer, when the fiber grating is to be fabricated by irradiating with UV converged by the cylindrical lens, the optical fiber bearing the coat layer is preferably placed in a position between the cylindrical lens and a focal point of the cylindrical lens so that the entire optical fiber including the coat layer can be positioned within a beam pattern of the UV converged toward the focal point by the cylindrical lens.
Since the irradiation density of the UV is the highest at the focal point to which the UV is converged by the cylindrical lens, when the UV irradiation is conducted not through the coat layer but with the coat layer removed, the glass part from which the coat layer is removed is disposed as close as possible to the focal point. However, when the UV irradiation is conducted through the coat layer as in this invention, the coat layer is locally damaged to be burnt or changed in color (yellowed) by disposing the coated fiber on the focal point. When the coated fiber is disposed so as to place the entire coat layer within the beam pattern, however, the coated fiber can be uniformly irradiated at the predetermined irradiation density without damaging the coat layer.
In particular, the coated fiber is most preferably placed not only within the beam pattern but also in a position where the outer face of the coat layer is internally in contact with the outer edge of the beam pattern. Specifically, the position for attaining the internal contact corresponds to a position closest to the focal point and a position for the entire coat layer placing within the beam pattern. Therefore, the UV can be irradiated at the highest irradiation density within the range where the damage of the coat layer is avoided. In this manner, the time required for writing the grating can be shortened, resulting in improving the efficiency of fabrication of the fiber grating.
The core is preferably loaded with hydrogen before the UV irradiation, or preferably co-doped with at least Sn in addition to Ge.
In another method of fabricating a fiber grating of this invention, a grating is written in a state where tensile strain is caused by applying a tensile force. Thereafter, when the applied tensile force is released so as to elastically restore the tensile strain, the grating pitch of the grating written in a core is narrowed correspondingly to the elastically restored tensile strain. Thus, the wavelength characteristic can be shifted toward a shorter wavelength, and in addition, the shift toward a shorter wavelength can be stably kept.
Specifically, according to the invention, a tension application step of causing tensile strain along a fiber axial direction by previously applying a tensile force along the fiber axial direction to a portion for writing a grating of an optical fiber to be fabricated into a fiber grating; an irradiation step of writing the grating with a predetermined grating pitch along the fiber axial direction in the core of the optical fiber by irradiating, with UV, the optical fiber with the tensile force applied in the tension applying step; and a tension release step of shifting the grating pitch of the grating written in the core toward a shorter wavelength by releasing the applied tensile force after the irradiation step are executed.
In this case, tensile strain (expansion strain) is caused along the fiber axial direction in the core of the optical fiber in the tension application step, so as to keep the core to be expanded along the fiber axial direction. Next, the irradiation step is conducted under this condition, so as to write the grating with the predetermined grating pitch in the expanded core. Then, the tensile force applied to the optical fiber where the grating has been written is released in the subsequent tension release step, thereby elastically restoring the expansion strain. Accordingly, the grating pitch of the written grating is shifted to be narrowed. As a result, the wavelength of light reflected by the grating is shifted toward a shorter wavelength correspondingly to the narrowed grating pitch. Therefore, the shift control of the wavelength characteristic toward a shorter wavelength, which cannot be realized by a conventional tension applying method for keeping an applied tensile force after writing a grating, can be realized by the present method. Also, in this case, differently from the conventional tension applying method, the grating is written under application of the tensile force and the tensile force is released thereafter. Therefore, the grating shifted toward a shorter wavelength is stably formed in the core of the optical fiber under application of no load, and hence, the wavelength characteristic shifted toward a shorter wavelength of the fiber grating can be stably obtained. Moreover, as compared with the conventional tension applying method, there is no need to keep the applied tension in an individual optical fiber, and therefore, a fiber grating easily dealt with and controlled in the wavelength by tension application can be easily mass produced.
The optical fiber used in the fabrication of the fiber grating may be a non-coated fiber formed of a core and a cladding, namely, one excluding a coat layer, which does not limit the invention, and a coated fiber obtained by coating the non-coated fiber with a coat layer can be used. In this case, in order to transmit UV for effectively causing the photo-induced refractive index change in the core through the UV irradiation, the coat layer is preferably formed from a UV transmitting resin. When the UV irradiation is conducted through the coat layer, the tensile force applicable in the tension application step can be set to a large value, and as the value of the applied tensile force is larger, the tensile strain caused in the core can be larger so as to increase the elastic restoration obtained in the tension release step, namely, the narrowed grating pitch. As a result, the shift of the wavelength characteristic toward a shorter wavelength can be increased, so as to enlarge the range for the wavelength control. For example, since a non-coated fiber is generally broken when a tensile force corresponding to expansion of 1% is applied, the tension application step is conducted in a small tension region where a smaller tensile force is applied. However, since the expansion where a fear of fracture occurs is generally 6% in a coated fiber, when the coated fiber is used in the tension application step, a large tensile force corresponding to the expansion of at least 4% or more can be applied.
Furthermore, after applying and releasing the tensile force for the wavelength control, a screening step of conducting a screening test on the portion of the fabricated fiber grating where the grating has been written by applying a predetermined tensile force may be carried out. Thus, a test on the mechanical characteristic of the portion where the grating is written with the wavelength controlled, namely, a test on the strength and surface damage, can be conducted following the fabrication of the fiber grating, and this method is suitable for a mass production system for fiber gratings.
The screening step may be conducted subsequently to a general fiber grating fabrication method in which the irradiation step alone is conducted for writing a grating without conducting the tension application step and the tension release step. Thus, in the fabrication of a fiber grating, writing of the grating and the screening test for the fiber grating where the grating has been written can be carried out in continuous procedures, and this is also suitable for the mass production system for fiber gratings.
The fiber grating fabrication apparatus of this invention includes a UV irradiation system for irradiating an optical fiber with UV so as to write a grating with a predetermined grating pitch along a fiber axial direction in a core of the optical fiber; and a tension applying mechanism for causing, by temporarily applying a tensile force, tensile strain along the fiber axial direction in a portion of the optical fiber irradiated with the UV by the UV irradiation system.
The xe2x80x9cUV irradiation systemxe2x80x9d includes any apparatus and mechanism for writing a grating by irradiating, with UV and preferably a UV laser beam, the optical fiber over a range in the axial direction for writing the grating, and includes, for example, a laser source, a phase mask, a mechanism for changing a position irradiated with the laser beam and the like. Also, the xe2x80x9ctension applying mechanismxe2x80x9d may be realized by a structure for fixing the positions of two portions of the optical fiber sandwiching the portion irradiated with UV by using adhering or frictional means and for applying a tensile force to the portion irradiated with UV by moving one or both of the fixed portions along the axial direction.
The tension application step is conducted by the tension applying mechanism for applying and keeping the tensile force along the fiber axial direction to the portion irradiated with UV, and the irradiation step is conducted by the UV irradiation system for irradiating with UV with the tensile force applied. Thus, the grating can be written through the UV irradiation in the core where tensile strain along the fiber axial direction is caused by the tension application. Then, the tension release step is carried out by releasing the applied tensile force by the tension applying mechanism after writing the grating. In this manner, the core shrinks along the fiber axial direction to restore, and the written grating shrinks in the fiber axial direction in accordance with the shrinkage of the core, resulting in narrowing the grating pitch as compared with that initially written. As a result, the wavelength of light reflected by the grating can be shifted toward a shorter wavelength than that reflected by the initially written grating.
The xe2x80x9ctension applying mechanismxe2x80x9d may be specifically constructed from a pair of fixing means for fixing two portions of the optical fiber away from each other in the fiber axial direction and sandwiching the portion irradiated with UV by the UV irradiation system; and moving means for forcedly moving at least one of the pair of fixing means away from and toward the other along the fiber axial direction. More specifically, an example of the xe2x80x9cfixing meansxe2x80x9d is a winding reel for winding the optical fiber around an axis perpendicular to the fiber axial direction so as to fix the optical fiber by using friction against the optical fiber. Specifically, by winding the optical fiber around the circumference of the winding reel, the optical fiber is fixed so as not to relatively move along the axial direction by using frictional resistance caused between the optical fiber and the circumference of the winding reel. In this case, the winding reel to be moved by the moving means is rotatably supported around the perpendicular axis in one position along the fiber axial direction, and the xe2x80x9cmoving meansxe2x80x9d is constructed from a motor for forcedly rotating, by predetermined revolutions, the winding reel to be moved with the optical fiber wound. Since the optical fiber is fixed in its position by using the frictional resistance, the optical fiber can be fixed with no fear of damage caused in the optical fiber itself, and when the winding reel is forcedly rotated by the motor around the center axis thereof, a tensile force can be easily and definitely applied to the optical fiber. The xe2x80x9cwinding reelxe2x80x9d is in a cylindrical or a column shape, and from the viewpoint of easiness in controlling the tension application, the xe2x80x9cmotorxe2x80x9d is preferably a pulse motor or the like whose revolutions can be determined in proportion to the number of input pulses.
Moreover, in using the fiber grating fabrication apparatus, when the irradiation step and the screening step alone are to be conducted, the writing of the grating and the screening test of the fiber grating where the grating has been written can be both carried out in the same fabrication apparatus. Specifically, after writing the grating, through the UV irradiation by the UV irradiation system, in the optical fiber under application of no load, a tensile force for causing predetermined expansion strain along the fiber axial direction for the screening test is applied by the tension applying mechanism.